Method of hardening wear surfaces and product



1966 1- F. ZLOTEK ETAL 3,

METHOD OF HARDENING WEAR SURFACES AND PRODUCT Filed Nov. 18, 1963INVENTORS THADD EU S F. ZLOTE ATTO R NEYS rats The present inventionrelates to an improved method of heat treating and hardening wearsurfaces, and to the product of such a method. The product is hereintypically shown and referred to as a high carbon steel or steel alloysprag to be used as a wedging strut between concentric races of anoverrunning clutch, and more especially a clutch subject to very highspeed overruns. However, while a hardened sprag furnishes a good exampleof the advantages of the invention in point of method and product, it isto be understood that the invention is not at all limited to thisparticular type of production. Since the result of the method is toprovide a chromized type of external surface layer or case which isextremely hard and wear resistant (substantially more so than aconventionally chromized case), the method is well adapted to theproduction of high speed bearing races or surface plates, rollers andramps of a roller type overrunning clutch, and other usages involvinghigh pressure and/or relative velocity factors.

Generally considered, the invention provides a method in which the spragor other object to be treated is, in preparation for a subsequent andknown type of chromizing or chromallizing, surface hardening treatment,subjected to a nitriding or carbonitriding heat treat operation; and inwhich the subsequently chromized product (after a usually necessaryrestoration of its body hardness because of annealing occasioned inchromizing) possesses a surface hardness and wearability characteristicfar superior to that found in products conventionally treated by a knownchromium diffusion procedure.

More specifically, an object and result of a combined preparatorynitriding in an ammonia-laden atmosphere, or following any othersuitable known procedure for producing nitrides, and subsequentchrornizing is the provision of a surface layer or case on the objectwhich has all the advantages of the conventional chromium carbide caseover one produced by electrodeposition or other film or coatingdepositing operation, in regard to its integral bond with the body basemetal, its being proof against peeling, spelling or flaking, itsresistance to corrosion and abrasion, etc. Yet it differs materiallyfrom the result of plain chromizing in that the diffused chromium layeror case itself possesses an intermediate chromium nitride stratum orphase. This intermediate phase (thickness-wise) in the otherwiseessentially chromized layer, as the result of the initial heat treatmentin an ammonia atmosphere, imparts an extreme degree of surface hardnessand wearability to the object, especially well suiting the method to thesprag production mentioned above, for certain speed and load operatingconditions.

Conventional chromizing or chromallizing procedure as practiced todayare contemplated for that phase of. the present method. One of these isa powder pack chrome diffusion or infusion operation in which iron orsteel pieces to be treated are packed in a powder containing a ohromiumhydro'halide refractory diluent composition effective upon heating in afurnace to not only produce the chrome diffusion but also to purge thefurnace of air and maintain it air-free throughout the chromizingtreatment. The furnace is heated to an elevated temperature which willbe a function of the time duration of the treatment, and when gaseousdiffusion has been continued sufficiently long to produce the desiredthickness aterrt of layer or case, theparts are permitted to coolslowly. The resulting annealing effect ordinarily requires a laterrestorative heat treatment to re-harden the body of the part, but thisis without effect on the chromized layer. The patent to Samuel No.2,536,774 of January 2, 1951 relates to a chromizing treatment of thissort, in which the chromium carbide takes an integral bond with theferrous or alloyed ferrous base metal. Samuel No. 2- 962,391 of November2-9, 1960 is another.

Another more recent development in the chromiz-ing procedure isillustrated and described in the patent to Samuel et al. No. 3,021,321of February 13, 1962. In this method chromium is diffused into thetreated object from a source, as distinguished from a powder pack,having a predetermined chromium-halide concentration. The metallurgicaleffect is essentially the same as in the powder pack method. Samuel2,962,391, disclosing the use of various hydrozine hydroh-alides,mentions the suitability of both methods. A chromium diffusiblecomposition may also be coated on the object.

As indicated, either of these chromizing procedures may be employed inthe second chromizing phase of the present improved method of theinvention; and it is also to be understood that other known ehromizingprocedures not incompatible with the objectives may be resorted to.

A specific objective of the invention is to provide a method in which asteel or alloy steel having a high carbon content of above 0.4%, such asSAE 52100, a chrome alloy, is the material of the sprag or other objectto be treated, since a carbon content of less than this concentrationhas been found to lead to increasing ductility of the case afterchromizing, rather than hardness. Such product is, following throughcleaning, heat treated to introduce the nitride component, and,following certain incidental operations (such as tumbling to removeburrs, grinding to provide desired formations, and the like) ischromized to produce a surface layer or case of the desired depth, say0.0003". It is then reh-ardened to restore its body or core to desiredhardness for its uprpose.

It is notable that whereas a sprag of given metallic composition, aschromized without the preparatory nitriding or carbonitriding treatment,exhibits a surface hardness of 12001300 VPN, a similar sprag compositionpreliminarily nitrided or carbonitrided, then chromized and re-hardenedunder the same chromizing and rehardening specifications, has a surfacehardness of well over 1500 VPN. This we attribute to the internalchromium-nitride modification of the primary chromized external layer atthe intermediate phase referred to above.

The foregoing as well as other objects will become more apparent as thisdescription proceeds, especially when considered in connection with theaccompanying drawing illustrating the invention, wherein:

The figure is a microphotograph of a nital etched section of an SAE52100 alloy steel sprag at a magnification of 2000 X.

In general, the combined nitriding-chromizing method of the invention, aspecific embodiment or adaptation of of which will be hereinafterdescribed in greater detail, is as follows:

A. Using as an example an extruded sprag stock of high carbon steel oralloy steel, the stock is cut to the desired sprag lengths, which arethoroughly cleaned to eliminate all surface contamination, and areheated in a furnace at a sufiicient temperature (200 F.-250 F.) to driveoff all moisture present.

B. The sprags are placed in baskets in a not excessive depth and areplaced in a heat treating furnace, such as an Ipsen type 400 or 800, thefurnace having been properly stabilized in temperature before loading.They are then heat treated under a specification calling for thepresence of raw ammonia, or any other means of introducing nitrogen intothe steel, at a temperature in the general range of 1535 F.1550 F., fora soaking time at temperature depending upon the desired body hardnesscharacteristic of the steel, for example 90 to 100 minutes.

C. Quenching and further cooling follow under controlled conditions, anddrawing to relieve stress.

D. The sprags or other products are then conventionally chromized, asdescribed in either of the patents to Samuel et al. identified above, orany other known chromizing or chromium diffusion procedure notinconsistent or incompatible with the objectives of the presentinvention.

E. Hardening follows to restore body hardness of the object lost byannealing in the chromizing treatment which, per se, is to be understoodnot to be a contribution of the present invention, except as performedsubsequent to nitriding, as described.

The above steps of course have interspersed therewith and/ or precedentor subsequent thereto, as in the production of sprags and other specialobjects, certain steps or manipulations not bearing upon the positiveheat treat aspects of the invention, such as tumbling to remove burrs orto polish, grinding to produce notches or to finish (other than at aworking surface), inspection, and the like. These will be referred to inthe detailed production specification.

It is contemplated that the nitride generating setting in which thepreparatory heat treating is performed may also be a carbon-heavyatmosphere, such as to produce a concentration of carbide nodulesadjacent to a chromized surface case or layer of the finished product.This ordinarily might be considered to be a poor heat treating practice,but it may be desirable in some instances in the present comprehensivenitriding-chromizing method, for such carbide inner layer forms a hardcrust to support stably the chromized layer modified by the intermediatechromium nitride phase referred to above.

Referring to the figure, the reference numeral generally designates thebase, body or core of the typical SAE 52100 steel instanced above. Witha 1.0% carbon content, and as treated in a furnace atmosphere of highcarbon potential of, say, 1.5% C, the core 10 exhibits a relativemultiplicity of complex iron chromium carbide particles or nodules 12 ina fairly close concentration adjacent the external, modified chromizedlayer or case 14, to be described, extending therefrom into the steelcore. These have the effect of affording an inner nodular supportingcrust for the last named layer which is referred to above. However, ifpreliminary nitriding is performed in a relatively carbon-neutralpotential, the occurrence of such ferrous carbide particles is lesspronounced. Such would be the situation were the carbon potential of theheat treating atmosphere reduced below 1%, the carbon content of thesteel in question, and with no differential causing carbon to migrate tothe steel structure.

The modified chromized layer, generally designated 14, includes a basiciron chromium carbide solid solution 16, traversed along its approximatemidpoint by the intermediate chromium nitride complex layer 18, whichhas a distinctly contracting, darker appearance, in a pink shade, asappears in a color microphotograph of the nitaletched section. Thereference numeral simply designates a plastic mount for the section ofthe figure, employed in the polishing and etching of the surface.Whatever the mechanical effect of this intermediate nitrided interlayeror phase may be, its effect on the overall chromized layer case 14 is tosubstantially increase its hardness and wearability, particularly withthe inner core strengthened by the iron-chromium carbide complex 12.

The following is a typical production heat treat specification for SAE52100 sprags, including incidental mechanical and inspectional steps notnecessarily called for at all in other types of production:

(a) Cut to length, cleaned of surface contamination and heated at 200F.250 F. to drive off all moisture.

(b) Placed in open basket (Ipsen type recommended) 2- in depth notsubstantially exceeding 1 /2" for sufficient heat soak and subsequentthorough quench. Place in heat treat furnace (Ipsen 400 or 800 type)after conditioning same as to temperature and atmosphere.

(c) If run at high heat previously, furnace temperature should bebrought down for /2 hour and stabilized before lOading sprags. Heattreating temperature to be maintained above the lower criticaltemperature, for instance 1535 F.l550 F. Dew point of atmospheremaintained at 10 :1 point. Atmosphere to consist of endothermic gas(200-225 cubic feet per hour), raw natural gas (40 cubic feet per houror more) and raw ammonia (9 cubic feet per hour :1 cubic foot).

(d) Soak at temperature 10() minutes and quench in oil (Park type AAA)at 90 F. minimum to 130 F. maximum, and air cooled slowly in circulatingair.

(e) Draw, after standing at room temperature for 3-4 hours, at 325F.-350 F. to stress relieve only (for loads less than pounds a minimumof 2 hours).

(f) Inspect for hardness, tumble to remove burrs, grind to notch, tumbleto'remove burrs for chromizing treatment and/ or inspect for surfacefinish.

(g) Chromize to 0.0003" case.

(h) Inspect as to depth of nitride-modified chromized depth.

(i) Harden to correct chromize-induced anneal.

(j) Inspect for modified chromize depth and hardness, tumble, andinspect surface finish, final dimensions, and magnaflux structure.

In regard to the condition of the furnace at the commencement of nitrideheat treating, it need not be cleared of a carbon-heavy atmosphere atthis time, and it is in fact desirable in certain instances to processimmediately after a carburizing operation in the furnace, therebylessening the required furnace time under certain conditions. If thesprags are processed in a relatively neutral atmospheric condition froma prior heat, more time in the furnace will be required for properprocessing. A dew point of +25 or lower will maintain a carbon potentialof 1.00% in the furnace atmosphere, and as indicated at (c) above, it isdesirable in a commercial production treatment to maintain the dew pointat approximately 10. The dew point should be checked before the work isplaced in the furnace and before removal. Not in substantial excess of100 pounds loading of sprags should be had in a single basket.

The quench oil should be Within the 90 F. F. temperature range beforequenching, and proper control should be maintained during the quench sothat F. is not exceeded. Air cooling should commence only after thecomplete mass reaches a stabilized oil temperature below 150 F., and aircooling should continue down to 75 F.

Room temperature having been reached, the nitride heat treated spragsshould have a Rockwell hardness of 93 to 94 on the 15N scale.

Item (d) states the temperature soak time for a hardened steel. However,should it be desired to produce an annealed steel, the soak time may bereduced under the same conditions; and this duration is also acceptablefor producing hardened steel sprags, provided the furnace is incarburizing condition from a previous load just prior to treatinganother.

In regard to the restorative hardening heat treatment (i) afterchromizing, this may be done under any suitable specification, but ingeneral a specification involving conditions similar to those of thenitriding or carbonitridmg heat treatment. That is, the sprags arecleaned of foreign surface contamination, are heated to drive offmoisture, are heat treated in similarly limited load weights and depths,but in a furnace cleared of all heavy carbon or ammonia atmosphere.Typical conditions of temperature and atmosphere: Dew point35 to 40,endothermic generator-35 :5 points (no ammonia) at 1525 F.1550 F. for aminimum of 60 minutes, similarly quenched in oil, air cooled and drawnat 300 F.320 F.

The fully and finally treated sprag will exhibit the following physicalproperties:

Core hardness60 minimum Rockwell C, 90 minimum Rockwell N;

Surface hardness (measured flat surface)Knoop hardness 1350 with 100gram load;

On micro-structure examination, an intermediate phase 18 of thicknessapproximately 20% of total thickness of modified chromized case 14 andcomplex nodular carbides proceeding immediately below case 14 into steelof core.

The chromium nitride modified case or layer 14 has, characteristicallyof a chromium diffusion layer, an integral bond with the base steel suchas prevents peeling, chipping or flaking apt to occur inelectrodeposited films and the like. It is extremely hard and wearresistant for the purposes of the present invention, but may, if a steelless than 0.4% carbon is treated, have a greater degree of ductilitysuiting it for other applications. It is resistant to corrosion andabrasion. In regard to maintenance of dimensional stability the methodhas the same advantage as before, and variation is readily predictable.

The nature of the nodular crust concentration at 12 is believed to be aniron chromium carbide complex, which may come from the components of theparticular alloy itself, or entirely or in lesser part from thechromized layer 14.

The initial heat treatment in a nitrogen including atmosphere, or in acarburizing nitriding atmosphere, involves little more than is involvedin conventional carburizing heat treatment, so that the cost of thecombined nitridingchromizing operation approximates the overall cost ofheat treatment of a product lacking the preparatory nitriding phase. Inother respects, the improved method and product possess all of theadvantages attributable to a simple chromizing procedure, plus thosespecial and notable advantages in respect to hardness which stem fromthe intermediate, chromium nitride phase of the modified chromiumcarbide layer.

What we claim as our invention is:

1. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment,quenching the article and then subjecting the article to a chromizingheat treatment to produce a chromized external layer on the articlemodified by a chromium nitride phase.

2. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment at atemperature above the lower critical temperature, and then subjectingthe article to a chromizing heat treatment to produce a chromizedexternal layer on the article modified by a chromium nitride phase.

3. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment in anatmosphere including nitrogen and carbon, quenching the article, andthen subjecting the article to a chromizing heat treatment to produce achromized external layer on the article modified by a chromium nitridephase.

4. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment in anatmosphere including nitrogen and carbon in a percentage content of thelatter at least as great as that of the article, and then subjecting thearticle to a chromizing heat treatment to produce a chromized externallayer on the article modified by a chromized nitride phase, with aniron-chromium carbide layer adjacent said layer extending into the coreof the article.

5. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment in anatmosphere including nitrogen and carbon in a percentage content of thelatter at least as great as that of the article, quenching the article,and then subjecting the article to a chromizing heat treatment toproduce a chromized external layer on the article modified by a chromiumnitride hase intermediate the thickness of said layer, with a ferrouscarbide-rich layer adjacent said layer extending into the core of thearticle.

6. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment in ahigh carbon potential and nitride producing atmosphere, quenching thearticle, subjecting the article to a chromizing heat treatment toproduce a chromized external layer on the article modified by a chromiumnitride phase intermediate the thickness of said layer, and heattreating the article to restore the hardness of said core as annealed bythe chromizing treatment.

7. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment at atemperature above the lower critical temperature in an atmosphereincluding nitrogen and carbon in a percentage content of the latter atleast as great as that of the article, quenching the article, subjectingthe article to a chromizing heat treatment to produce a chromizedexternal layer on the article modified by a chromium nitride phaseintermediate the thickness of said layer, with an iron-chromium carbidelayer adjacent said layer extending into the core of the article, andheat treating the article to restore the hardness of said core asannealed by the chromizing treatment.

8. A wear-resistant article having a ferrous metal body provided with anintegrally bonded external chromized case modified by a phase ofchromium nitride.

9. A wear-resistant article having a steel body provided with anintegrally bonded external diffusion case modified by a phase ofchromium nitride intermediate the thickness of said case.

10. A wear-resistant article having an alloy steel body provided with anintegrally bonded external diffusion case modified by a phase ofchromium nitride.

11. A wear-resistant article having a ferrous metal body provided withan integrally bond-ed external diffusion case modified by a phase ofchromium nitride intermediate the thickness of said case, with a ferrouscarbide-rich concentration directly inwardly adjacent said case.

12. A wear-resistant article having a steel body provided with anintegrally bonded external diffusion case modified by a phase ofchromium nitride intermediate the thickness of said case, with aniron-chromium carbiderich concentration directly inwardly adjacent saidcase.

13. A Weararesistant article having a steel body provided with anintegrally bonded external chromized case modified by a phase ofchromium nitride intermediate the thickness of said case, with aniron-chromium carbide-rich concentration directly inwardly adjacent saidcase.

14. A method for the surface hardening treatment of ferrous articles,comprising subjecting the article to a nitriding heat treatment, andthen subjecting the article to a chromizing heat treatment to produce achromized external layer on the article modified by a chromium nitridephase.

15. The method of claim 14, in which the nitriding treatment is carriedout in a nitrogen-rich atmosphere having in excess of 1% carbon content,for a ferrous article having, say, about 1.0% carbon content.

16. The method of claim 14, in which the nitriding treatment is carriedout in a nitrogen-rich atmosphere having about 1.5% carbon content, fora ferrous article having, say, about 1.0% carbon content.

17. The method of claim 14, in which the nitriding treatment is carriedout in a nitrogen-rich atmosphere, for a ferrous article having, say,about 1.0% carbon content, at a temperature of the order of 1535 F.-1550F. and for a time period inversely related in duration to the percentageof carbon content of the nitriding atmosphere.

18. The method of claim 14, in which the nitriding 7 8 treatment iscarried out in a nitrogen-rich atmosphere temperature of the order of1535 F.1550 F. and for having in excess of 1% carbon content, for aferrous a time period of the order of 90-100 minutes.

article having, say, about 1.0% carbon content, at-a temperature of theorder of 1535 F .1550 F. and for a References Clted by the Exammer timeperiod inversely related in duration to the percent- 5 UNITED STATESPATENTS age of carbon content of the nitriding atmosphere. 2,048,2767/1936 Marlies et a1. 29195 19. The method of claim 14, in which thenitriding 3,010,856 11/1961 Seelig et a1 117107.2 X

treatment is carried out in a nitrogen-rich atmosphere having in excessof 1% carbon content, for a ferrous DAVID RECK Prlmary Examme" articlehaving, say, about 1.0% carbon content, at a 10 C. N. LOVELL, AssistantExaminer.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,282,746 November 1, 1966 Thaddeus F. Zlotek et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 1, lines 32 and 60, for "chromallizing", each occurrence, read"Chromallizing" Signed and sealed this 18th day of February 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. A METHOD FOR THE SURFACE HARDENING TREATMENT OF FERROUS ARTICLES,COMPRISING SUBJECT TO A NITRIDING HEAT TREATMENT, QUENCHING THE ARTICLEAND THEN SUBJECTING THE ARTICLE TO A CHROMIZING HEAT TREATMENT TO